Lowering the dielectric constant of an interlevel insulating film is one of important problems to be solved for fabricating the semiconductor device that is operating faster and reduces its power consumption. In this regard, various ideas have been implemented with a view toward lowering a dielectric constant.
As for semiconductor devices in the related-arts, the following methods (1) to (3) have been already proposed:                (1) a method for doping fluorine to a silica film, which is an inorganic dielectric film (as described in, e.g., JP-A-11-241117);        (2) a method for forming an organic dielectric material having a low dielectric constant as a base material; and        (3) a method for forming a porous film intentionally.        
However, in the case of the method as defined in the method (1), a silica film might be doped with fluorine on the order, at most, of a few percentages in element proportion, due to the restriction being placed by the deterioration problem caused by the moisture resistance of the dielectric film. Therefore, the dielectric constant of silica-based interlevel dielectric film formed by the method (1) might be lowered in limited range, such as from 10% to 15%, as compared with that of being formed in conventional ways.
In the case of the method as defined in the method (2), the heat resistance and mechanical strength might be considerably deteriorated even as compared with those formed by the silica-based interlevel dielectric film by the conventional art, because of organic material as the base material, from which this method is likely to lead to a problem of deteriorating reliability of a semiconductor device.
In the case of the method as defined in the method (3), due to randomness of the porous structure, the mechanical strength of the interlevel dielectric film might be deteriorated remarkably. As the result, the semiconductor device becomes fragile when it is being packaged, which is also ended up being low reliability of the semiconductor device.
Furthermore, since most of the porous films formed (or prepared) by the method (3) do not have a closed porous structure, the moisture resistance of the interlevel dielectric film might be lowered remarkably, which also causes the semiconductor device to decrease its reliability.
As mentioned above, either by the methods of the related-art or the conventional arts, the dielectric film always encounters the problem of difficulty of lowering a dielectric constant and/or a problem of insufficient mechanical strength.